Cellular expanded articles formed by molding gas-charged and pre-expanded thermoplastic beads, the so-called particle foams, are widely utilized as thermal/sound/impact insulation, packaging, sport and cushioning materials for reasons of low density, better insulation characteristics, optimal energy absorption compared to solid polymers. The molding possibility of foamed beads in combination with their relatively small sizes enables either a production of simple configurated parts like blocks/plates or particularly, as another advantage in comparison to extruded semi-finished parts, a flexible forming of complex articles like e.g. 3D parts. In recent years, the use of molded particle foams have grown in automotive applications such as bumper impact absorber, seat cores, and floor mat leveling material.
Up to now, the widest materials of particle foams are represented by polystyrene (PS) and polyolefin (PP, PE or their copolymers). In general, expanded articles made of gas-charged PS beads are called as EPS, particle foams formed by polypropylene beads are known as EPP and the one made of polyethylene is named as EPE.
Expanded polystyrene (EPS) has taken its place nowadays as an important material in e.g. insulation, in construction applications and as a packaging material for a wide range of industrial applications and food industry. However, the method applied to produce EPS particle foams is rather complicated and expensive, mainly due to a number of, partially time-consuming, process steps involved (Eaves, D.: Handbook of Polymer Foams, Rapra Technology, 2004; Britton, R : Update on Mouldable Particle Foam Technology, Smithers Rapra, 2009):                Production of unexpanded PS beads charged with an organic blowing agent and typically in size of 0.1-2.0 mm,        Pre-expansion of said beads by using steam,        Maturing of pre-expanded EPS beads for a period of several hours (often overnight or even longer),        Molding and further expansion of matured beads.        
Production of unexpanded polystyrene beads can be implemented either by suspension polymerization or by extrusion process. The predominant production route is via a suspension polymerization yielding a range of sizes of spherical beads. These are charged with a volatile organic blowing agent (often a mix of isomers of pentane) in the final stage before dewatering and drying, then followed by an organic coating to prevent agglomeration in the later processes.
The extrusion processes produce “microbeads” of uniform size directly from a melt of polystyrene (which may already contain a blowing agent) by use of an underwater micropelletiser. The polystyrene melt is supplied from an extrusion line, or even directly from a polymerization reaction carried out in a series of static mixers and melt pumps. The subsequent pre-expansion processing of PS beads is precisely same both to polymerized and extruded beads. The pre-expansion process involves using steam to heat and agitate the beads either in a batch or in a continuous process. As the beads are warmed by the steam to above the glass transition temperature of the material, they soften and the blowing agent boils at a large number of nucleation points, forming cells which grow so that the whole bead is foamed throughout, where the key variables in prefoaming are the steam pressure (temperature), the amount of dilution air and time.
The pre-expansion results yet in a vacuum inside the beads due to the rapid expansion of bead size. This may cause impairment or even collapse of cell structure inside the beads, in consequence of the generally insufficient mechanical strength of polystyrene. Thus, the pre-foamed EPS beads need to be matured, i.e. an atmospheric pressure inside the beads needs to be created by a permeation of air into them. In this process stage, the beads, now called as a “prepuff”, are blown through pipes to large silos, where they are dried and discharged and allowed to mature for a period of several hours (often overnight or even longer), allowing them to cool and the cell walls to become rigid, able to support the negative pressure once the residual blowing agent has condensed. In the maturing progresses, air diffuses into the beads and they become stable enough to be processed further. It is well known that for a given bead type two or more expansion stages are necessary to achieve a lower final density. In case of multi-stage expansion, maturing is required between expansions.
Molding is also affected with steam—the prepuff beads are blown into an aluminum mold and steam applied through a number of small vents. This softens the beads and expands them further, using the residual blowing agent which remains in them after the prefoaming and ageing steps, in order to fuse adjacent beads together. Vacuum may be applied to the mold in the later stages, to help create a well fused surface on the molding. The distribution of steam between the mold halves can be adjusted to optimize the molding process, prevent distortion, etc. Cooling follows before the molding can be ejected and allowed to dry.
In addition to the complicated, time-consuming and expensive production processes, PS as a thermoplastic polymer shows some deficiencies like brittleness, insufficient impact strength, swelling when moist, too high compressibility, a generally low mechanical level, poor mounting properties, relatively high water vapor permeability etc. that are not favored when it comes to building insulation and its manufacturing (US 2011/0171456; Domininghaus, H.: Die Kunststoffe und ihre Eigenschaften, 6. Auflage, Springer Verlag, 2005). Furthermore, the lacking thermal stability or low temperature resistance resulted from the relatively low service temperature of either 65-80° C. for long-term or 80-90° C. for short-term as well as the poor resistance to chemicals such as organic solvents and fuel (Domininghaus, H.: Die Kunststoffe und ihre Eigenschaften, 6. Auflage, Springer Verlag, 2005) limit or even eliminate some applications of EPS in e.g. insulation for water heating appliances, automotive or microwave-related packaging.
Expanded polyolefin beads are another important particle foam in the beads family. It is known that the volume of moldable beads produced from polyolefins is very much smaller than that of polystyrene, even though foamed polyolefins have some significant property advantages (Domininghaus, H.: Die Kunststoffe und ihre Eigenschaften, 6. Auflage, Springer Verlag, 2005). Among the particle foams of polyolefins, expanded polypropylene (EPP) plays a very important role, since EPP achieves an even better property profile compared to some other polymeric foams such as EPS, EPE as well as PU. Molded articles of EPP are generally characterized by properties like excellent impact energy absorption, good toughness, small residual deformation, better temperature stability, good chemical resistance and very low water vapor permeability.
EPP already applies within the area of packaging for industrial goods and in the automotive branch. A great variety of products, like protection for side impacts, sun visors, column and door covers, tool boxes and bumper inserts are made of this material.
However, common polypropylene grades normally feature a linear chain structure, having thus a sharp melting transition and low melt strength (particularly in extension), which is responsible for a cell structure being difficult to control, or even makes an expansion impossible. Production of EPP beads requires then often an introduction and use of an expensive high melt strength (HMS) resin which is a modified grade having a long chain branching (US 2011/0171456; Domininghaus, H.: Die Kunststoffe and ihre Eigenschaften, 6. Auflage, Springer Verlag, 2005).
Moreover, the processes for preparing and molding the EPP beads are complicated and comprised of (Eaves, D.: Handbook of Polymer Foams, Rapra Technology, 2004; Britton, R: Update on Mouldable Particle Foam Technology, Smithers Rapra, 2009):                Converting the polymer, very often HMS polypropylene or a blend containing such HMS PP, into micropellets by e.g. extrusion,        Impregnation of the PP micropellets with a hydrocarbon blowing agent such as propane at elevated temperature (e.g. 130-160° C.) and pressure for several hours,        Expansion of the impregnated micropellets to form low density beads with particle size of some 4-5 mm after the pressure is released,        Molding the expanded beads to final articles by compressing and fusing them with help of steam and backpressure,        Post-ageing of expanded polypropylene (EPP) products in an autoclave, often required to achieve a full stability.        
Disadvantageous in the process chain of EPP is the fact that the blowing agent impregnated into the beads can not be held there long at ambient pressures for later expansion. Instead, once impregnated with a blowing agent, the beads must be expanded immediately, or held under high pressure. Another disadvantage is the rapid outgasing of the blowing agent out of polypropylene beads, which is essentially complete already within a few days following the impregnation, i.e. the expanded beads need to be molded immediately or very soon after the expansion process. Further importantly, expanded beads have a high bulk, so that transporting them (and the finished products) is costly, or producing and molding the beads are required to take place on the same site. All this, use of chain branching grade, complex process, rapid outgasing and high transport costs etc., impedes the polyolefin bead foams in the marketplace (US 2011/0171456; Domininghaus, H.: Die Kunststoffe und ihre Eigenschaften, 6. Auflage, Springer Verlag, 2005).
Considered as one of the most disadvantageous weaknesses, PP is known to be very vulnerable to oxidative degradation under the influence of elevated temperature and/or sunlight because of the existence of tertiary carbon atoms. Such degradation is recognized as a free-radical chain reaction, which leads to chain scission. The addition of stabilizers has been widely used to depress this radical reaction. However, it is difficult to maintain the long-term performance of stabilizers for various reasons, including volatility (Pielichowski, K.,et al: Thermal Degradation of Polymeric Materials, Rapra Technology Limited, 2005). This weakness confines the outdoor applications or use of PP, including expanded PP beads, in an oxidative environment.
Polyalkylene terephthalates, belonging to the polyester family, particularly polyethylene terephthalate (PET) as a commodity thermoplastic resin, are mechanically strong in terms of strength, stiffness and hardness, chemical-resistant (much more resistant to most chemicals compared to PS), and show good thermal stability arisen from a high service temperature of either 100° C. for long-term or 200° C. for short-term and from a high Vicat softening temperature (VST/B120) of 185° C. (Domininghaus, H.: Die Kunststoffe und ihre Eigenschaften, 6. Auflage, Springer Verlag, 2005).
As described in details of (US 2011/0171456), PET or polyalkylene terephthalate have shown to be basically suitable and in some aspects even superior in comparison to EPS for e.g. building industry requirements:                PET shows the compatibility in massive form with mineral based building materials such as concrete, clay or minerals etc.        The stability and structural strength of massive PET have been used for the casting of concrete parts.        The use of terephthalate foams in building and construction for windows sills or as insulation against heat bridging in the building industry are claimed in some patents. Such foams can bear some weight load of other construction elements.        Foamed terephthalate can provide the structural integrity showing properties being resistant versus compression by weight (compression strength) in combination a) with resistance to creeping, flowing or destructive shearing (compressive modulus and shear strength) and b) with a reasonable level of remaining shear elongation.        
A more important character of polyalkylene terephthalate such as PET is its less vapor permeation in comparison to PS (the most important foamable material for insulation till now) (Domininghaus, H.: Die Kunststoffe und ihre Eigenschaften, 6. Auflage, Springer Verlag, 2005), since the vapor content in a foamed polymer is known for its negative impact on the thermal conductivity. EPS may become less effective in the insulation property with the time of an outdoor utilization.
PET is well-known for the excellent gas barrier ability (Domininghaus, H.: Die Kunststoffe und ihre Eigenschaften, 6. Auflage, Springer Verlag, 2005), which results in a slow outgasing and is thus considered as a big benefit in comparison to PP. As mentioned above, expanded polypropylene has to be finally expanded and molded soon after the gas impregnation.
The above described property profile makes, therefore, polyalkylene terephthalate attractive as one of the materials suitable to be processed to gas-charged and expanded cellular beads, which can be formed by molding. Such expanded beads of PET can be termed E-PET, similar to EPS or EPP.
Among the polyalkylene terephthalate family, the low-viscous PET resins like bottle-grades, fiber-grade or post-consumer materials are pricely competitive and attractive for foaming process. Two PET grades now dominate the global market, i.e. fiber-grade PET and bottle-grade PET. These standard grades differ mainly in molecular weight or intrinsic viscosity (IV), respectively, optical appearance and the production recipes. Textile fiber-grade PET has an IV of between 0.55 and 0.67 dl/g, while bottle-grade PET, appearing “glass-clear” in the amorphous state, shows an IV of between 0.75 and 1.00 dl/g (Scheirs, J., et al: Modern Polyesters: Chemistry and Technology of Polyesters and Copolyesters, John Wiley & Sons, 2003).
However, expansion of polyalkylene terephthalate to produce low density cellular materials by using a physical blowing agent and extrusion process has been found to be a difficult process, mainly for the reason that these polymer resins are required to have appropriate rheological characteristics such as high melt strength and high (intrinsic and melt) viscosity.
EP0866089 describes e.g. an extrusion foaming of PET resins, which typically represents a free expansion. During the free expansion, the extrudate released from a die is depressured and the growing of initiated bubbles in size (extrudate expansion), forced by the vapor pressure of the blowing agent, which is generally higher than atmospheric pressure, is not restrained in the atmosphere, except by the melt strength of the molten resin. In case of a gas-charged melt mixture featuring insufficient melt strength, it is either not able to shape an extrudate or the built bubbles collapse when leaving the die exit. EP0866089 claims a foamability of PET resins which have to be solid state upgraded before, known as solid state polycondensation (SSP), in presence of pyromellitic dianhydride to reach an intrinsic viscosity of 0.85-1.95 dl/g, preferably between 1.00 and 1.95 dl/g. The intrinsic viscosity is in correlation with melt strength, thus essential for foamability of a polyester resin during the free expansion in a foam extrusion process. Both comparative examples of EP0866089 demonstrate that the starting resins of PET characterized by IV values of 0.80 and 0.92 dl/g are not foamable in a melt extrusion process, thus a free expansion, due to the lacking melt strength.
In addition, pre-expanded beads of polyalkylene terephthalates are not easily mold-formed due to their relatively high melting temperature and an equally high crystallinity under the temperature conditions necessary for a mold forming.
However, U.S. Pat. No. 6,306,921 claims expanded PET beads obtained from aromatic polyester resins having a melt strength of 1 cN at 280° C., a melt viscosity of more than 1500 Pa·s at 280° C. and with shear rate tending to zero, an intrinsic viscosity of more than 0.80 dl/g and a crystallization rate by heating at 120° C. for 5 minutes so that the resulting crystallinity is not higher than 15%. Resins having the indicated characteristics are obtained by solid state upgrading of the polymer in the presence of PMDA and in a temperature range of 150 to 210° C.
The preparation of the foamed beads is performed by hot cutting the foamed threads, by means of rotating blades at the output of an extrusion head having multiple holes, according to U.S. Pat. No. 6,306,921. Hence, the foaming process of PET resins disclosed both in EP0866089 and U.S. Pat. No. 6,306,921 belongs identically to the category of free expansion, which requires a high melt strength preventing the gas-charging melt from a cell collapse. As indicated in the description and comparative examples of EP0866089, foaming of PET resins with IV value of 0.92 dl/g or less is not possible in case of a free expansion. The claim of U.S. Pat. No. 6,306,921 that foamed PET beads are obtainable from polyester resins having an intrinsic viscosity of only more than 0.80 dl/g are, therefore, not convincing. This was also confirmed by the current invention (s. Comparative examples 3 and 4 of this invention), where a foaming and granulation method being similar to U.S. Pat. No. 6,306,921 were applied, and by the examples of U.S. Pat. No. 6,306,921, where PET copolymer with a melt strength of 150 cN at 280° C., a melt viscosity of 1800 Pa·s at 300° C. and an intrinsic viscosity of 1.25 dl/g (obtained by solid state upgrading a copolymer having an initial IV of 0.63 dl/g in presence of 0.4% PMDA) is foamed for production of the beads.
On the other hand, it is very much necessary to employ an extrusion head with tiny orifices to obtain small or micro-sized beads in case of a free expansion. The examples of U.S. Pat. No. 6,306,921 disclose the use of the multiholes having a diameter of 0.1 mm The tiny hole diameter causes, however, a very high shear rate: With 90 kg/h throughput through 24 holes, the average shear rate is estimated to be over 450′000 /s, assuming even a melt density of about 1400 kg/m3 in the temperature range of 260-300° C. and at a melt pressure of 110 bar. The above estimation illustrates how much the melt mixture may be sheared or shear degraded at the extrusion head during the bead production with hot cutting. This again requires use of polymer resins having high viscosity (possibly IV>1.0 dl/g) in the bead preparation of U.S. Pat. No. 6,306,921.
Besides, the upgrading process is indeed a complicated and highly cost-/time-consuming procedure: The granules containing PMDA undergo a solid state polycondensation at 210° C. for 10 h (in general, 24 h may be necessary to upgrade polyester resins from an IV below 0.80 dl/g to 1.25 dl/g).
U.S. Pat. No. 6,306,921 further claims the foamed PET beads characterized by a density between 30 and 500 kg/m3, a melt strength of more than 1 cN at 280° C., a melt viscosity of more than 1500 Pa·s at 280° C. etc.
However, the E-PET beads of U.S. Pat. No. 6,306,921 show disadvantageously a poor cell structure according to the inventors: While the outermost layer is characterized by microcells of 50 to 500 μm, the center part of the beads features a macrocell structure with cell sizes of a few millimeters.